The study of oligonucleotides is a key area of research for many academic and industrial laboratories. See S. Agrawal Trends in Biotechnology 1996, 14, 375-382; J. Man Drug Discovery Today 1996, 1, 94-102; and W. Rush Science 1997, 276, 1192-1193. The therapeutic and diagnostic potential of oligonucleotides has sparked a substantial amount of research activity. One important application of oligonucleotides is the ability to modulate gene and protein function in a sequence-specific manner. However, many research efforts are hampered by the small quantities of oligonucleotides that are available for study. A method to produce large quantities of oligonucleotide compounds having high purity would greatly facilitate oligonucleotide research. Furthermore, it would be highly useful to be able to prepare derivatives of certain oligonucleotides. However, the synthesis of oligonucleotides and their analogs is often a tedious and costly process.
RNA is generally synthesized and purified by methodologies based on the following steps: phosphoramidite coupling using tetrazole as the activating agent, oxidation of the phosphorus linker to the diester, deprotection of exocyclic amino protecting groups using NH4OH, removal of 2′-OH alkylsilyl protecting groups using tetra-n-butylammonium fluoride (TBAF), and gel purification and analysis of the deprotected RNA. Examples of chemical synthesis, deprotection, purification and analysis procedures are provided by Usman et al. in J. Am. Chem. Soc. 1987, 109, 7845; Scaringe et al. in Nucleic Acids Res. 1990, 18, 5433-5341; Perreault et al. in Biochemistry 1991, 30, 4020-4025; and Slim and Gait in Nucleic Acids Res. 1991, 19, 1183-1188. Odai and coworkers describe reverse-phase chromatographic purification of RNA fragments used to form a ribozyme. See Odai et al. FEBS Lett. 1990, 267, 150-152. Unfortunately, the aforementioned chemical synthesis, deprotection, purification and analysis procedures are time consuming (10-15 min. coupling times), subject to inefficient activation of the RNA amidites by tetrazole, incomplete deprotection of the exocyclic amino protecting groups by NH4OH, limited by the low capacity of RNA purification using gel electrophoresis, and further limited by low resolution analysis of the RNA by gel electrophoresis. Therefore, the need exists for improved synthetic processes for the synthesis of oligonucleotides.
One important class of oligonucleotide analogues are compounds that have a phosphorothioate in place of the phosphodiester linkage. Phosphorothioate analogues are important compounds in nucleic acid research and protein research. For example, phosphorothioate-containing antisense oligonucleotides have been used in vitro and in vivo as inhibitors of gene expression. Site-specific attachment of reporter groups onto the DNA or RNA backbone is facilitated by incorporation of single phosphorothioate linkages. Phosphorothioates have also been introduced into oligonucleotides for mechanistic studies on DNA-protein and RNA-protein interactions, as well as catalytic RNAs.
Introduction of phosphorothioate linkages into oligonucleotides, assembled by solid-phase synthesis, can be achieved using either an H-phosphonate approach or a phosphoramidite approach. The H-phosphonate approach involves a single sulfur-transfer step, carried out after the desired sequence has been assembled, to convert all of the internucleotide linkages to phosphorothioates. Alternatively, the phosphoramidite approach features a choice at each synthetic cycle: a standard oxidation provides the normal phosphodiester internucleotide linkage, whereas a sulfurization step introduces a phosphorothioate at that specific position in the sequence. An advantage of using phosphoramidite chemistry is the capability to control the state of each linkage, P═O vs. P═S, in a site-specific manner. The earliest studies to create phosphorothioates used elemental sulfur, but the success of the phosphoramidite approach is dependent on the availability and application of more efficient, more soluble sulfur-transfer reagents that are compatible with automated synthesis. Therefore, the need exists for novel sulfur-transfer reagents that are compatible with automated oligonucleotide synthesis.
Another important class of oligonucleotides is double-stranded RNA which can be used to initiate a type of gene silencing known as RNA interference (RNAi). RNA interference is an evolutionarily conserved gene-silencing mechanism, originally discovered in studies of the nematode Caenorhabditis elegans (Lee et al, Cell 75:843 (1993); Reinhart et al., Nature 403:901 (2000)). It is triggered by introducing dsRNA into cells expressing the appropriate molecular machinery, which then degrades the corresponding endogenous mRNA. The mechanism involves conversion of dsRNA into short RNAs that direct ribonucleases to homologous mRNA targets (summarized, Ruvkun, Science 2294:797 (2001)). This process is related to normal defenses against viruses and the mobilization of transposons.
Double-stranded ribonucleic acids (dsRNAs) are naturally rare and have been found only in certain microorganisms, such as yeasts or viruses. Recent reports indicate that dsRNAs are involved in phenomena of regulation of expression, as well as in the initiation of the synthesis of interferon by cells (Declerq et al., Meth. Enzymol. 78:291 (1981); Wu-Li, Biol. Chem. 265:5470 (1990)). In addition, dsRNA has been reported to have anti-proliferative properties, which makes it possible also to envisage therapeutic applications (Aubel et al., Proc. Natl. Acad. Sci., USA 88:906 (1991)). For example, synthetic dsRNA has been shown to inhibit tumor growth in mice (Levy et al. Proc. Nat. Acad. Sci. USA, 62:357-361 (1969)), is active in the treatment of leukemic mice (Zeleznick et al., Proc. Soc. Exp. Biol. Med. 130:126-128 (1969)); and inhibits chemically-induced tumorigenesis in mouse skin (Gelboin et al., Science 167:205-207 (1970)).
Treatment with dsRNA has become an important method for analyzing gene functions in invertebrate organisms. For example, Dzitoveva et al. showed for the first time, that RNAi can be induced in adult fruit flies by injecting dsRNA into the abdomen of anesthetized Drosophila, and that this method can also target genes expressed in the central nervous system (Mol. Psychiatry. 6(6):665-670 (2001)). Both transgenes and endogenous genes were successfully silenced in adult Drosophila by intra-abdominal injection of their respective dsRNA. Moreover, Elbashir et al., provided evidence that the direction of dsRNA processing determines whether sense or antisense target RNA can be cleaved by a small interfering RNA (siRNA)-protein complex (Genes Dev. 15(2): 188-200 (2001)).
Two recent reports reveal that RNAi provides a rapid method to test the function of genes in the nematode Caenorhabditis elegans; and most of the genes on C. elegans chromosome I and III have now been tested for RNAi phenotypes (Barstead, Curr. Opin. Chem. Biol. 5(1):63-66 (2001); Tavernarakis, Nat. Genet. 24(2):180-183 (2000); Zamore, Nat. Struct. Biol. 8(9):746-750 (2001).). When used as a rapid approach to obtain loss-of-function information, RNAi was used to analyze a random set of ovarian transcripts and have identified 81 genes with essential roles in C. elegans embryogenesis (Piano et al., Curr. Biol. 10(24):1619-1622 (2000). RNAi has also been used to disrupt the pupal hemocyte protein of Sarcophaga (Nishikawa et al., Eur. J. Biochem. 268(20):5295-5299 (2001)).
Like RNAi in invertebrate animals, post-transcriptional gene silencing (PTGS) in plants is an RNA-degradation mechanism. In plants, this can occur at both the transcriptional and the post-transcriptional levels; however, in invertebrates only post-transcriptional RNAi has been reported to date (Bernstein et al., Nature 409(6818):295-296 (2001). Indeed, both involve double-stranded RNA (dsRNA), spread within the organism from a localized initiating area, to correlate with the accumulation of small interfering RNA (siRNA) and require putative RNA-dependent RNA polymerases, RNA helicases and proteins of unknown functions containing PAZ and Piwi domains.
Some differences are evident between RNAi and PTGS were reported by Vaucheret et al., J. Cell Sci. 114(Pt 17):3083-3091 (2001). First, PTGS in plants requires at least two genes—SGS3 (which encodes a protein of unknown function containing a coil-coiled domain) and MET1 (which encodes a DNA-methyltransferase)—that are absent in C. elegans, and thus are not required for RNAi. Second, all of the Arabidopsis mutants that exhibit impaired PTGS are hyper-susceptible to infection by the cucumovirus CMV, indicating that PTGS participates in a mechanism for plant resistance to viruses. RNAi-mediated oncogene silencing has also been reported to confer resistance to crown gall tumorigenesis (Escobar et al., Proc. Natl. Acad. Sci. USA, 98(23):13437-13442 (2001)).
RNAi is mediated by RNA-induced silencing complex (RISC), a sequence-specific, multicomponent nuclease that destroys messenger RNAs homologous to the silencing trigger. RISC is known to contain short RNAs (approximately 22 nucleotides) derived from the double-stranded RNA trigger, but the protein components of this activity remained unknown. Hammond et al. (Science 293(5532):1146-1150 (August 2001)) reported biochemical purification of the RNAi effector nuclease from cultured Drosophila cells, and protein microsequencing of a ribonucleoprotein complex of the active fraction showed that one constituent of this complex is a member of the Argonaute family of proteins, which are essential for gene silencing in Caenorhabditis elegans, Neurospora, and Arabidopsis. This observation suggests links between the genetic analysis of RNAi from diverse organisms and the biochemical model of RNAi that is emerging from Drosophila in vitro systems.
Svoboda et al. reported in Development 127(19):4147-4156 (2000) that RNAi provides a suitable and robust approach to study the function of dormant maternal mRNAs in mouse oocytes. Mos (originally known as c-mos) and tissue plasminogen activator mRNAs are dormant maternal mRNAs that are recruited during oocyte maturation, and translation of Mos mRNA results in the activation of MAP kinase. The dsRNA directed towards Mos or TPA mRNAs in mouse oocytes specifically reduced the targeted mRNA in both a time- and concentration-dependent manner, and inhibited the appearance of MAP kinase activity. See also, Svoboda et al. Biochem. Biophys. Res. Commun. 287(5):1099-1104 (2001).
The need exists for small interfering RNA (siRNA) conjugates having improved pharmacologic properties. In particular, the oligonucleotide sequences have poor serum solubility, poor cellular distribution and uptake, and are rapidly excreted through the kidneys. It is known that oligonucleotides bearing the native phosphodiester (P═O) backbone are susceptable to nuclease-mediated degradation. See L. L. Cummins et al. Nucleic Acids Res. 1995, 23, 2019. The stability of oligonucleotides has been increased by converting the P═O linkages to P═S linkages which are less susceptible to degradation by nucleases in vivo. Alternatively, the phosphate group can be converted to a phosphoramidate or alkyl phosphonate, both of which are less prone to enzymatic degradation than the native phosphate. See Uhlmann, E.; Peyman, A. Chem. Rev. 1990, 90, 544. Modifications to the sugar groups of the oligonucleotide can confer stability to enzymatic degradation. For example, oligonucleotides comprising ribonucleic acids are less prone to nucleolytic degradation if the 2′-OH group of the sugar is converted to a methoxyethoxy group. See M. Manoharan ChemBioChem. 2002, 3, 1257 and references therein.
Therefore, the need exists for improved synthetic processes that facilitate the synthesis of oligonucleotides. Representative examples of needed improvements are better activating agents for phosphoramidite coupling of nucleotides, better sulfur-transfer reagents for preparing phosphorothioate-containing oligonucleotides, and improved procedures for purifying oligonucleotides.